How do you simplify (8x^4/3y-3)^4
Since it appears to be an exercise in exponents, I assume that you meant 3y^-3.
So, raise everything to the 4th power:
8^4x^4^4/3^4y^-3^4
= 4096x^8/81y^-12
= 4096 x^8 y^12 / 81
If you really did mean 3y-3, then you'd end up with
4096 x^8 / 81(y-1)^4